• Multicystic dysplastic kidney is the most frequent cause of an abdominal mass in the neonate, but controversy continues as to the optimal management of these lesions, since little is known about their natural history. Experience with two complicated cases and a review of reports of retained multicystic dysplastic kidneys suggest that such lesions pose a significant risk to their hosts. Malignancy, reversible hypertension, pain, and mass effect have been associated with retained lesions. Infection is another potential hazard that is frequently cited but poorly documented in the literature. In light of the currently low morbidity and mortality associated with operation and anesthesia in the neonatal period, resection appears to be the treatment of choice for the neonate with a multicystic dysplastic kidney.